package cn.jingyuan.bee.utils.codec;

/**
 * 凯撒密码实现<br>
 * 算法来自：https://github.com/zhaorenjie110/SymmetricEncryptionAndDecryption
 */
public class Caesar {

    /** 26 个字母表 */
    public static final String TABLE = "AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz";

    /**
     * 传入明文，加密得到密文
     *
     * @param message 加密的消息
     * @param offset 偏移量
     *
     * @return 加密后的内容
     */
    public static String encode(String message, int offset) {
        final int len = message.length();
        final char[] plain = message.toCharArray();
        char c;
        for (int i = 0; i < len; i++) {
            c = message.charAt(i);
            if (!Character.isLetter(c)) {
                continue;
            }
            plain[i] = encodeChar(c, offset);
        }
        return new String(plain);
    }

    /**
     * 传入明文解密到密文
     *
     * @param cipherText 密文
     * @param offset 偏移量
     *
     * @return 解密后的内容
     */
    public static String decode(String cipherText, int offset) {
        final int len = cipherText.length();
        final char[] plain = cipherText.toCharArray();
        char c;
        for (int i = 0; i < len; i++) {
            c = cipherText.charAt(i);
            if (!Character.isLetter(c)) {
                continue;
            }
            plain[i] = decodeChar(c, offset);
        }
        return new String(plain);
    }

    /**
     * 加密轮盘
     *
     * @param c 被加密字符
     * @param offset 偏移量
     *
     * @return 加密后的字符
     */
    private static char encodeChar(char c, int offset) {
        int position = (TABLE.indexOf(c) + offset) % 52;
        return TABLE.charAt(position);

    }

    /**
     * 解密轮盘
     *
     * @param c 字符
     *
     * @return 解密后的字符
     */
    private static char decodeChar(char c, int offset) {
        int position = (TABLE.indexOf(c) - offset) % 52;
        if (position < 0) {
            position += 52;
        }
        return TABLE.charAt(position);
    }

}
